ISAM error:illegal argument to ISAM function - SQL Reports Designer

[martinkent]

I am using Reports Designer and having problems when summarising certain fields.

I am wanting to select agents from multiple skills but only want to show one record for each agent. I am selecting data from the CAGENT table and want to create the following statement:
[blue]select LOGID, WORKMODE, min(AGTIME)
from cagent
where ACD=1 and (SPLIT=2 or SPLIT=3 or SPLIT=9 or SPLIT=29)
group by LOGID, WORKMODE[/blue]

but this returns the "ISAM error:illegal argument to ISAM function" error.

If I just run it for the agents it works OK:
[green]select LOGID, min(AGTIME)
from cagent
where ACD=1 and (SPLIT=2 or SPLIT=3 or SPLIT=9 or SPLIT=29)
group by LOGID[/green]

Can anyone explain why and suggest a fix?

Thanks